Transmission not shifting 2000 silverado 2500

ok, well i had the issue with my guages, i just ended up re-wireing them in because the feed off the fuse box was putting out bearly any voltage, so i thought that was messing up my truck not shifting. but basicly i have no overdrive or 3rd, well im assuming no third but i may, if i was to go 50 my truck will be rev'd up to around 300-3500 rpm's. now i know people with similar issues have said it could be a ignition switch, now is there a simple way to test/fix a bad electrical part on the ignition switch? also the wire i replaced was the ign 1 fuse box wire.

its not a clutch issue, and it stoped shifting when the guages started to die out, i hard wired gauges just so i wouldnt loose tach or speed due to fact that with tranny not shifting i wanted to keep good eye on all my gauges, its something electrical because it will shift into 3rd or overdrive randomly but kicks itself out of it basicly right after shifting into it

its not a clutch issue, and it stoped shifting when the guages started to die out, i hard wired gauges just so i wouldnt loose tach or speed due to fact that with tranny not shifting i wanted to keep good eye on all my gauges, its something electrical because it will shift into 3rd or overdrive randomly but kicks itself out of it basicly right after shifting into it

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How many miles are on it? You'r trany is 12 years old honestly if I where you I would just replace it but that is just me.

Could be a lot of things vss...drac..speedo..tach..abs...maf..tps...map...ecm...bad wire in harness ...bad ground..ignition switch.
brake switch ..list goes on. Need to scan it for codes will sure help get you in right direction .

Some customers may comment that the vehicle has no movement when the transmission is shifted to DRIVE or THIRD position, but there is normal operation when it is shifted to SECOND, FIRST or REVERSE position.Cause

This condition may be caused by a damaged forward sprag assembly (642).Correction

When inspecting the sprag, it is important to test the sprag for proper operation by holding the outer race (644) with one hand while rotating the input sun gear (640) with the other hand. The sun gear should rotate only in the counterclockwise direction with the input sun gear facing upward. If the sprag rotates in both directions or will not rotate in either direction, the sprag elements should be inspected by removing one of the sprag assembly retaining rings (643). Refer to SI Unit Repair section for forward clutch sprag disassembly procedures.If the sprag is found to be damaged, make repairs to the transmission as necessary. A new forward roller clutch sprag assembly is now available from GMSPO.[TABLE]
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